1. Field of the Invention
The present invention relates to an imaging device formed as a video camera, and particularly to an imaging device that carries out imaging with combination of a mechanical shutter and an electronic shutter.
2. Description of the Related Art
In an imaging device employing a frame-transfer (FT) charge coupled devices (CCD) image sensor as its imaging element, the image area (light receiving part) is shielded from light during signal charge readout. Furthermore, in an imaging device employing a complementary metal oxide semiconductor (CMOS) image sensor, the image area is shielded from light during charge readout in order to achieve simultaneity of signal charge accumulation.
A scheme of using a rotary shutter is known as a method for shielding the image area from light. The rotary shutter is obtained by providing a shutter blade that is driven to rotate by a motor between an imaging lens and the imaging element. The rotary shutter is so configured as to be capable of setting the state in which the optical path is blocked and the state in which the optical path is not blocked depending on the rotational angular position of the shutter blade.
Furthermore, at the timing of the state in which the shutter blade blocks the optical path and light is not incident on the light receiving part of the imaging element, a signal accumulated in the light receiving part is read out. Thereby, unnecessary exposure of the FT CCD image sensor in the middle of the transferring of the light reception signal is prevented. Details of the state of the linkage between the rotation of the shutter blade and the drive timing of the imaging element will be described later in the description of an embodiment of the present invention.
Japanese Patent Laid-Open No. 2006-308841 (Patent Document 1) discloses an example of a shutter device that controls the period of the incidence of light to the imaging plane of a solid-state imaging element and the period of blocking of light to the imaging plane.
Japanese Patent Laid-Open No. Sho 54-108631 (Patent Document 2) discloses an example in which three shutter blades are employed to widen the variable range of the light-blocking range.
If light blocking and exposure for an imaging element are alternately carried out by rotating one shutter blade, a problem arises that the timing of the light blocking is settled in a somewhat fixed manner and the flexibility in the shutter speed, which corresponds to the accumulation period of the charge for obtaining the image signal of one frame, is limited.
Specifically, e.g. the following case is assumed. A shutter blade is so configured that light is blocked in a 180°-angle range of the blade and light is not blocked in the remaining 180°-angle range. In addition, the shutter blade is rotated one revolution per one frame. In this case, the maximum period during which the imaging element can receive light within one frame is a period about half the one-frame period, and it may be impossible to expose the imaging element for a period longer than this maximum period.
It is possible to extend the exposure period within the one-frame period by setting the angle range of light blocking by the shutter blade smaller than 180°. However, if the angle range of light blocking by the shutter blade is set smaller, the period during which light is blocked within the one-frame period is also correspondingly shortened, which causes a possibility that the light-blocking period is insufficient to read out the signal obtained in the imaging element.
In particular, in recent years, there are an increasing number of occasions where photographing is carried out by a photographic method called variable speed ramping with this kind of video camera. The variable speed ramping refers to a way in which photographing is carried out with the frame rate (frame per second, hereinafter referred to as the FPS) smoothly changed differently from photographing with a fixed number of frames per second, such as 30 frames per second for video for television broadcasting and 24 frames per second for video for a film.
For example, if a walking person is photographed by the variable speed ramping and the obtained video is reproduced with a fixed number of frames, sharp video representation is possible, such as video representation in which this person walks slowly when waking on the far side but quickly passes on the near side suddenly. A user is often allowed to change the FPS in real time by operating a user interface (UI) such as a jog dial. In addition, e.g. a scheme of programming a FPS change curve in advance and executing the program is also employed.
In such variable speed ramping, the shutter blade is often an obstacle depending on the setting of the cycle of the imaging.
There is a desire for the present invention to allow an imaging element to be favorably shielded from light by a mechanical shutter while keeping the flexibility in the frame cycle, the shutter speed, and the exposure time.